The present invention is directed to coating formulations for the preparation of transfer elements. More specifically, the present invention is directed to water-based transfer element coating formulations, processes for making transfer elements with water-based formulations, and processes for using the transfer elements thus prepared. In one embodiment, the transfer element coating formulations of the present invention comprise a wax-in-water emulsion, an oil, an aqueous polymer emulsion, a colorant, an optional water-soluble leveling agent, and an optional inert filler, said coating formulations containing substantially no volatile organic compounds.
The term "transfer element" generally refers to a class of materials employed in printing processes, including both impact printing and nonimpact printing processes, such as carbon paper, typewriter ribbons, thermal transfer ink donor films, electro-resistive ribbons, and the like. Generally, a transfer element is an element, such as a sheet or a ribbon, comprising a substrate on which is coated a colored material such as an ink or a dye, which colored material is capable of being transferred from the substrate or donor sheet onto a receiver sheet, such as paper or transparency material, to form a visible image. Transfer of the colored material from the substrate to the receiver sheet can be by any suitable method, such as the application of heat to the substrate in the instance of thermal transfer printing, the application of pressure to the substrate in the instances of conventional typewriting, other impact printing processes, and carbon paper, the application of current in the instance of electro-resistive printing processes, and the like.
Thermal printing is a nonimpact printing process that enables formation of high resolution images. These printing processes are simple, offer low noise levels, and are very reliable over extended usages. Thermal printing processes may be classified into three categories. Direct thermal printing entails the imagewise heating of special papers coated with heat sensitive dyes, such that an image forms in the heated areas. Another method of thermal printing is known as the dye transfer or dye sublimation technique, and operates by heating a donor sheet coated with a sublimable dye. When the donor sheet is imagewise heated, the dye sublimates and migrates to the receiver sheet, which possesses a polymeric coating into which the dye diffuses, forming an image. A third method of thermal printing is known as thermal transfer printing. The thermal transfer printing process entails imagewise heating of a donor sheet containing ink, which donor sheet is in intimate contact with the heater on one side and with the receiver sheet on the other side. Imagewise heating of the donor sheet affects the ink in such a way as to cause it to transfer from the donor sheet to the receiver sheet, thereby resulting in image formation. Thermal transfer printing methods generally employ uncoated plain papers, which enables prints with acceptable appearance and excellent archival properties. In addition, the thermal transfer printing method may be employed for color printing applications by using ink donor sheets of the desired color or colors.
Thermal transfer printing processes generally employ a thermal print head, an ink donor sheet, and a receiver sheet. The side of the donor sheet containing the ink is placed in contact with the receiver sheet, and heat originating from the print head is then applied to the donor sheet. Heat conducted through the donor sheet increases the temperature of the ink, which may cause it to melt, soften, decrease in viscosity, or otherwise undergo a transition that enables at least some of the ink to transfer to the receiver sheet. After the receiver sheet and ink donor sheet are separated, an image remains on the receiver sheet. An alternative method of heating the ink donor sheet, known as resistive heating, employs an array of electrodes instead of thermal print head to generate a current between the electrodes and a grounded conductive layer in the ink donor sheet. This method is described in the IBM Journal of Research & Development, Vol. 29, No. 5, 1985, the disclosure of which is totally incorporated herein by reference.
Impact printing processes also employ transfer elements. For example, typewriters depend on transfer elements to make prints. In this instance a key strikes a ribbon or donor roll which is brought into contact with a sheet of paper or other receiver sheet such as a label, envelope or office form. The impact energy of such contact causes ink or other colored material on the transfer element to transfer to the receiver sheet, thereby forming a printed character in the shape of the raised surface of the key. Another type of impact printer in common use is the so-called dot matrix printer. These printers work like typewriters, using impact from a key to transfer colored material from a donor ribbon to a receiver sheet, except that a single key, comprising a matrix of fine pins which are electronically arranged to form characters, is used in place of the many individual keys of the typewriter. Yet another type of impact printing process employs carbon paper. In this instance, pressure is applied to the top sheet in a set of sheets wherein the surfaces of at least some of the sheets are coated with an ink or other colored material, thereby causing the ink or colored material to transfer from the coated sheet to the sheet in contact with the coating.
Many types of transfer elements are known for impact and other printing processes. For example, fabric ribbons consist of a roll of cloth which has been soaked with a specially formulated ink, a controlled amount of which is released to the receiver sheet during the printing process. This type of ribbon has been in use for more than 100 years. It is a multiple use ribbon in that the same area of the ribbon can be repeatedly struck to give prints of acceptable optical density until the ink becomes depleted. While fabric ribbons have mostly been replaced with new types of ribbon for typewriter applications, because of their robustness, they are commonly used in dot matrix printers.
Single strike ribbons afford a much denser and uniform print than a fabric ribbon. They generally comprise a densely colored thin coating on a plastic substrate which is entirely transferred to a receiver sheet during the printing process. Single strike correctable ribbons are a specially formulated version of single strike ribbons which have been designed to form a print which can be removed from the receiver sheet by a process involving abhesion to a sticky surface of a correction tape. Single strike correctable transfer elements are described in, for example, U.S. Pat. No. 3,825,437, U.S. Pat. No. 3,825,470, U.S. Pat. No. 4,092,280, U.S. Pat. No. 4,161,551, and U.S. Pat. No. 4,260,664, the disclosures of which are totally incorporated herein by reference. The processes and formulations used to prepare single strike correctable transfer elements typically entail the use of organic solvents.
Multistrike ribbons comprise a spongy layer with ink trapped in the pores of the sponge. The spongy layer is coated on a thin plastic substrate. The printing process causes the expulsion of a controlled amount of ink from the spongy layer to the receiver sheet. Multiple strikes (typically up to about 6) can be made on the same area of the ribbon without loss in optical density. They are more economical to use than single strike transfer elements and generally result in better print quality than fabric ribbons.
Transfer elements are commonly manufactured by processes that entail either hot melt or organic-solvent based coating techniques. Hot melt coating, which is economically and environmentally attractive, has a disadvantage in that this coating method places viscosity and flow requirements on the formulation, which tends to restrict significantly the potential choices of materials in the formulation. For example, hot melt coating formulations generally contain low levels of pigment, do not contain high molecular weight polymers, and generally must contain ingredients that are soluble or readily dispersible in the melt. In addition, it is difficult to prepare uniform, very thin coatings of consistent quality by hot melt techniques. These disadvantages are significant drawbacks in that it is generally desirable to provide a uniform, highly pigmented coating on a very thin substrate, thus enabling a product that can provide maximum usage by incorporating a long, densely colored transfer element on a roll into a relatively small package.
Accordingly, organic solvent-based coating techniques are most frequently used to manufacture transfer elements. Organic solvent coating techniques, however, possess disadvantages in that they consume large volumes of solvents. For environmental reasons, these solvents must be recovered or incinerated under strict conditions, which greatly increases the cost of organic-solvent coating processes. In addition, the solvents themselves can be expensive, in some instances accounting for over 50 percent of the unit market cost of a coating.
Thus, there is a need for coating processes and formulations that do not have these disadvantages. The present invention is directed to transfer element formulations which comprise water-based emulsion coatings.
Transfer elements made according to the present invention are prepared from coating formulations containing substantially no volatile organic compounds. As used herein, the term "volatile organic compound" includes those organic liquids conventionally used in solvent coating operations, such as aliphatic hydrocarbons, branched and unbranched, typically with up to about 12 carbon atoms or more, aromatic hydrocarbons, such as benzene, toluene, xylene, and tetralin (tetrahydronaphthalene), aliphatic and aromatic alcohols, such as ethanol, isopropanol, butanol, amyl alcohol, benzyl alcohol, and the like, organic ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, benzophenone, and the like, halogenated aliphatic and aromatic compounds, such as chlorobenzene, carbon tetrachloride, chloroform, methylene chloride, bromobutane, dichlorodifluoromethane, and the like, organic ethers, organic amines, organic acids, organic esters, and any other organic compounds capable of evaporating to any significant degree under ambient and/or coating conditions and which can reside in the atmosphere. As used herein, the term "volatile organic compound" does not encompass materials with relatively low vapor pressures under ambient and coating conditions, such as the oil and polymer components of the coating formulations of the present invention or other organic compounds indicated herein as being possible coating formulation components. Typically, non-volatile organic compounds, including those contained in the coating formulations of the present invention, such as vegetable oils, refined rapeseed oil, mineral oils, and the like, have very low vapor pressures, typically less than about 1 millimeter of mercury at room temperature (20.degree. to 25.degree. C.) and high boiling point, typically about 300.degree. C. or more, at atmospheric pressures.
Coating formulations containing water are known. For example, U.S. Pat. No. 3,337,361 (La Count) discloses a process for making a pigmented pressure-sensitive transfer coating which comprises depositing on a water impervious base a pigmented pressure sensitive transfer coating containing practically no oily plasticizer and comprising a water slurry containing a pigment, a binder, and a lubricant which is substantially incompatible with the binder, which is miscible or dispersible in water, which is non-volatile, and which has a specific gravity greater than the binder, removing water from the coating to dry it, and then subjecting the dried coating to a heat treatment to melt and fuse it and to cause a thin film of the lubricant to form between the base and the remainder of the coating, thereby to constitute a transfer layer. The binder is a wax in emulsion form and the lubricant is selected from the group consisting of ethylene glycol and polyethylene glycol and having a specific gravity greater than the wax. This patent also discloses a process which comprises mixing a water slurry containing magnetic iron oxide and polyethylene glycol with a wax dispersion containing modified Fischer-Tropsch wax, emulsifiable polyethylene, ester wax, an anionic dispersing agent, and water, coating a base carrier with the mixture at room temperature, removing the water from the coating, and melting and fusing the remainder of the coating to form a liquid film of polyethylene glycol adjacent to the base.
In addition, U.S. Pat. No. 3,904,803 (Brown et al.) discloses prressure-sensitive reusable transfer elements of the squeeze-out type having a microporous resinous ink-releasing layer firmly bonded to a flexible foundation. The transfer element is characterized by a bonding undercoating layer applied to the foundation as an aqueous composition comprising a mixture of two water-dispersible resinous binder materials, one of which is water-soluble and does not insolubilize on drying, and the other of which is insoluble in water in that it forms a water-insoluble film on drying. According to the teachings of this patent, the water-base composition is coated on a flexible foundation such as paper and provides a bonding layer with excellent bonding properties for microporous ink layers applied to this bonding layer either from water or from organic solvent vehicles. U.S. Pat. No. 4,112,178 (Brown) also discloses pressure sensitive transfer elements comprising water-applied resinous base coating supporting a resinous ink-releasing layer, characterized by the undercoating consisting essentially of a water-dispersible, water-insoluble hydrophilic polyurethane resin.
Further, U.S. Pat. No. 3,925,273 (Cuthbertson et al.) discloses essentially aqueous stable oil-in-water printing emulsions suitable for use in transfer printing processes, particularly textile printing processes. The printing emulsions comprise a sublimable disperse dye, an anionic dispersing agent, a water soluble film forming resin, a non-ionic emulsifying agent, a hydrocarbon mineral oil with a boiling point of at least 290.degree. F. which is at least 50 percent distilled in the boiling range of 300.degree. F. to 450.degree. F., water, and a solid water soluble viscosity modifying agent. The emulsion has a viscosity in the range of 800 to 5,000 centipoise at 25.degree. C.
Additionally, U.S. Pat. No. 4,034,128 (Kelley) discloses a rheologically stable aqueous dispersion of metal-modified novolak resin particles prepared by grinding an aqueous mixture of the metal-modified novolak resin and anionic polymeric dispersing agent in the presence of a small amount of an organo-phosphorus compound containing two or more phosphonic acid or alkali metal phosphonate groups per molecule. Dispersions of the metal-modified resin particles so produced can be incorporated in color developing coating compositions containing a binder which may be applied and dried on a carrier paper to produce a pressure sensitive color developing record sheet. U.S. Pat. No. 4,363,664 (Delaney) also discloses stable concentrated free-flowing aqueous dispersion compositions containing one or more colorless dyestuff precursors and one or more surface active agents useful in the manufacture of paper for pressure sensitive carbonless duplicating manifold systems and thermal marking systems.
Further, U.S. Pat. No. 4,527,993 (Schuster et al.) discloses a process for producing transfer printing papers with foamed aqueous dyestuff liquors in which the consistency of the foam acts as a thickening agent, thereby enabling reduction of the moisture content of the print paste. The dyestuff-containing liquor is made finely porous by means of surfactants into stable foams, and the transfer carrier is printed in the desired pattern with the foam and dried.
In addition, U.S. Pat. No. 3,314,814 (Newman) discloses a method of preparing a transfer element wherein the transfer composition comprises a film-forming hydrophilic or water-soluble binder material, a non-volatile oily material which is not compatible with the film-former, and a quantity of imaging material applied to a water-resistant foundation in the form of a solution in a miscible water-aliphatic solvent mixture and solidified by evaporation of the solvent mixture to form a microporous structure containing within its pores a pressure-exudable ink containing the oleaginous material and the imaging material.
Also of collateral interest with respect to the present invention are U.S. Pat. No. 3,468,692, U.S. Pat. No. 3,472,674, U.S. Pat. No. 4,087,580, U.S. Pat. No. 4,168,338, and U.S. Pat. No. 4,324,817.
Although known formulations are suitable for their intended uses, a need continues to exist for transfer element coating formulations that are water-based and that avoid the difficulties encountered with transfer elements prepared by hot melt coating techniques or by organic solvent coating techniques. There is also a need for transfer elements that can be prepared by economically attractive methods. Further, there is a need for transfer elements that can be prepared by environmentally safe methods. A need also exists for transfer element coating formulations that can be employed to prepare transfer elements with high pigment loadings. Further, a need exists for transfer elements wherein the coating of transfer material is desirably thin. There is also a need for single-strike transfer elements that can be prepared from aqueous emulsions of the coating components. Additionally, there is a need for aqueous processes for preparing a wide variety of transfer elements, including single strike impact transfer elements, single-strike correctable transfer elements, single strike thermal transfer ink transfer elements, carbon papers, and the like. A need also exists for aqueous processes for preparing transfer elements with a wide variety of colorants, including water soluble dyes, oil soluble dyes, pigments, magnetic solids, and invisible taggants such as fluorescent compounds and infrared light-absorbing materials. Further, there is a need for methods for preparing transfer elements that employ no volatile organic solvents.